1. Field of the Invention
Graphite containing polymer material.
2. Related Art
In the prior art various forms of graphite material have been used to enable polymer material to conduct or dissipate electrostatic charge. One of the first such graphite containing materials added to polymers was carbon black which has an appropriate amount of electrostatic dissipative capacity, but is difficult to handle, relatively light and extremely time consuming to put in place.
Static electricity and electrostatic discharge (ESD) are naturally occurring phenomena. Simply stated static electricity is electrical energy at rest on a surface. It is generally created by the rubbing together and separating of two materials, one of which is usually non-conductive. Typically, one material gives up electrons and becomes positively charged; the other material takes on electrons and becomes negatively charged. ESD may be characterized as the sudden discharge of electrostatic potential from one body to another. A good example may be the shock one may receive when touching a metal door after walking across a carpeted floor.
In many environments, ESD may damage or destroy sensitive electronic components, erase or alter magnetic media, or set off explosions or fires in flammable environments. These discharges may be caused by a variety of sources; most commonly there is a direct discharge from a person or equipment into a sensitive object.
One way of preventing ESD is to reduce the generation of charges in the first place. A second way of preventing ESD is to provide a ground path for the safe dissipation of accumulated charges to ground before they can build up to a level that is harmful to sensitive equipment. A third method is to provide shielding or protection of devices and equipment from discharge through packaging. ESD may also be controlled with materials, such as conductive plastics, that do not generate high levels of charge, dissipate charges before they can accumulate at dangerous levels, or that provide electrostatic shielding to prevent charges from reaching the sensitive device.
Floors of clean rooms that need to have low electrostatic discharge potential for, as an example, manufacturing semiconductor devices, need to be conductive and lightweight. These floors require lightweight floor panels, because they tend to be suspended on vibration dampening mechanisms. A lightweight, conductive polymer that dissipates electrostatic charges before they build up to dangerous levels would be useful.
Other applications for ESD polymer materials are as computer covers and chip trays. Computer covers that offer ESD protection will protect delicate computer components on the inside from harmful electrostatic discharges. Chip trays are used to move and store semiconductor circuits during dicing, testing and packaging operations. These operations are generally when sensitive integrated circuits are most vulnerable to ESD. An electrostatic dissipative chip tray allows electrostatic charges to bleed off before they build up to dangerous levels that could harm an isolated integrated circuit chip.
U.S. Pat. No. 5,582,781 discloses, inter alia, a method of making graphite foam material. Expanded graphite is made from flexible graphite foil. The expanded graphite is then compounded into sheets to make insulation material.
U.S. Pat. No. 5,882,570 discloses, inter alia, a method of injection molding graphite and a thermoplastic material. This method also uses 45 to 60 percent by weight expanded graphite. The compound of thermoplastic material and re-expanded graphite is fed into a molding system (e.g. injection molding system) at relatively high temperature and injected into a mold where a plastic material is formed. The material is valuable for its heat conducting capacity for use in for example, thermal management.
Many parts of automobiles are painted using an electrostatic painting scheme. Electrostatic painting increases the uniformity with which the applied paint covers the part that is painted compared with plain spray painting. Paint in the form of small droplets or a fine powder is given an electrical charge, while the part to be painted is given the opposite charge. The charge differential impels the paint toward the part to be painted. When the paint touches the part the charge differential is neutralized allowing the paint to adhere to the part.
For this electrostatic paint scheme to work the part to be painted must be able to hold or dissipate a charge. When the part is a non-conductive polymer, a conductive primer must be coated over the part to enable the paint to uniformly cover the part. This primer generally contains a Volatile Organic Compound (VOC).
Emissions of VOCs have been curtailed by local, state and federal regulations. A limit on VOC emissions either eliminates the option of electrostatic painting of polymer parts, or is a limiting factor in the capacity of a painting operation that uses VOCs to paint polymers. A method that reduces or eliminates VOCs from the electrostatic paint scheme would increase the number of parts an operation could paint in a given time.
It is desirable to provide, at relatively low cost, compounds that may dissipate charges before they accumulate to dangerous levels, or allow themselves to be electrostatically modified.